1. Field of Invention
The present invention relates to a granular moving-bed apparatus, and more particularly to a two-stage granular moving-bed filter utilizing different kinds of granular filter media to clean up gas.
2. Related Art
Coal-fired power plants, or metallurgical processes and incineration plants generate high-temperature gas containing a large amount of fly ashes and dusts, sulfides, nitrides or other contaminants, and if the exhaust flue gas or raw syn-gas is emitted without processing, the environment of mankind will be affected seriously. In order to solve the environmental pollution problems due to hazardous gases, industrial countries have to constitute strict emission standards, and also input considerable research and development (R&D) resources to investigate how to remove the harmful pollutants in the gas streams effectively so as to conform to the emission standards.
In prior art, many methods of gas cleanup exist, among which some methods are common, namely mechanical separation of dust in cyclones, electrostatic dust collecting, baghouse filters, ceramic candle filters, granular moving-bed filters, etc. Cyclones are considered to be powerful and cheap pre-separators for gas cleanup purposes. Their removal efficiency is, however, limited to about 90% and rapidly deteriorates for particles smaller than 10 microns.
In the most widely used electrostatic dust precipitators, corona is utilized to ionize the exhaust gas so as to make the harmful materials carrying negative charges, and these materials are then caught on earthed collector plates to achieve the objective of gas cleanup. Electrostatic precipitators can be operated economically in flue gases of large volumes, but their efficiency is influenced by effects of chemical composition of particles, particle electric resistivity, moisture content, and temperature of gas.
Furthermore, another baghouse filter dust collecting technique also achieves the effect of gas cleanup through retaining the harmful materials in the exhaust gas, passing through the baghouse filter fabric. The baghouse filters offer very high dust collection efficiency and, operating in low temperature, they have the advantage over electrostatic dust precipitators, that the electric resistivity of dust particles does not play any role, making them competitive for high-resistivity ashes. Particles of different size are removed by different physical mechanisms in baghouse filters. The highest removal efficiencies are obtained for the large particles at high gas velocities and for the finest particles at low velocities. Baghouse filters can work at higher temperatures, depending on fabric materials. Ceramic materials, usually based on alumina, quartz or aluminum silicates are the best choice. A disadvantage when compared with electrostatic dust precipitators is the larger pressure drop and relatively low gas face velocity. Low gas face velocity gives rise to large filtration surface and inherently high costs.
Ceramic candle filters have been and are still being tested at full scale operation at several IGCC demonstration projects. Filter cleaning is done by backpulsing usually with nitrogen. Typical problems encountered during the testing are the breakage and strength degradation of ceramic candles owing to the overheating, thermal shocks and excessive pressure drop. In addition, the plenum vibration and back-pulse cleaning expose the candle filters to thermal and mechanical fatigue stresses that may ultimately lead to fracture of the filter elements.
Granular moving-bed filters are very well suitable for high temperature gas filtration. They are developed as key subsystems of current integrated gasification combined cycle (IGCC) and advanced pressurized fluidized bed combustion (PFBC) power generation systems. Their potential economic advantage is based on the fact that the granular moving bed filters may be more compact than either electrostatic precipitators or baghouse filters. The principal disadvantage of these filters is that either a very thick bed or very fine granular material (or both) are required to give high removal efficiency of particulates in the 0.5 to 10 microns size range. The requirement for thick beds results in large expensive equipment, while the use of very fine granular material causes high pressure drop, poor bed flow characteristics, accompanied by stagnant zones and hot spots in granular moving bed, causing corrosion and plaques on louver wall. Flow patterns in moving bed are influenced by friction of filter media on louver walls. It results in uprise of stagnant zones alongside louver walls. Besides particulate removal, granular moving beds may be able to capture other contaminants through the use of other materials (such as sorbent, activated carbon, etc.), in two stage process, but in one apparatus.
In order to improve the removal efficiency of granular moving bed filters, and extend the options in which the granular filter media can be applied, the technologically developed countries are devoted to research and development of environmental-friendly gas cleanup technologies, and so far, the granular moving-bed filter is a highly regarded gas cleanup apparatus, and can be considered as the exhaust gas cleanup apparatus widely used in the future. FIG. 1 is a schematic view of a common granular moving-bed filter. Referring to FIG. 1, the granular moving-bed filter is constituted by a plurality of hopper-shaped structures 16, and each hopper-shaped structure 16 has louver walls 10 and 11 at two sides thereof. The granular material, serving as the filter medium, enters from the top of the granular moving-bed, so that the granular material 15 goes down along the hopper-shaped structure 16 to clean up the harmful substances in the exhaust gas stream passing through the granular material, and the granular material which has adsorbed impurity or lost filtering effect exits from the bottom of the granular moving-bed. In FIG. 1, the raw syn-gas or exhaust gas flow 90 enters the granular moving-bed from the louver wall 10 at inlet side of the granular moving-bed, and passes through the filter medium 15. The harmful or polluting material contained in the exhaust gas flow 90 is filtered out by the filter medium 15, and the clean gas flow 91 exits from the louver wall 11 at the outlet side of the granular moving-bed. FIG. 2 is a schematic view of the granular moving-bed filter generating stagnant zones. Similarly, the exhaust gas flow 90 enters the granular moving-bed via the louver wall 10, and the clean gas flow 91 exits the louver wall 11. However, in the prior art, when flowing in the flow path, the granular material 15 generates kinematic friction with the louver walls 10 and 11, so that the granular flow generates a central fast-flowing zone 12, a quasi-stagnant zone 13, and a stagnant zone 14 in the flow path. When the stagnant zones are formed alongside the louver walls 10 and 11, because the granules are not moving, the dust filtered will deposit, and gradually plug the exhaust gas flow 90 from flowing through, thereby affecting the efficiency of filtering and generating the clean gas flow 91. As the same stagnant zone uprises on louver wall 11, the gas pressure drop in moving bed increases, resulting in an unacceptable value of pressure drop across the filter.
For example, in the prior art, U.S. Pat. No. 7,132,088 discloses a granular moving-bed apparatus, which utilizes filter medium flowing in granular moving-bed with internal saddle-roof-shaped flow-corrective elements, so that the granular flow is divided into two streams, characterized by mass flow without stagnant zones, and the problems of the corrosion of the louver walls or stagnant zones in the prior art can be solved by this mass flow design.
Furthermore, DE4030896 discloses a granular moving-bed filter, which utilizes two different moving-beds divided by a hardware structure to clean up exhaust gas. However, the path of the exhaust gas in moving bed is long and changing arrangement of flow from counter-current to co-current or vice versa, and thus the pressure drop of the exhaust gas is high, thereby energy-intensive and affecting the flow rate of the gas. Additionally, DE3817685 discloses a granular moving bed apparatus, which also utilizes two streams of different granular media kept apart by a perforated vertical wall to uniformly distribute and clean up the exhaust gas. In the technique, because the perforated vertical wall, separating the two streams of granules, may be plugged by dust coming with flue gas or by dust from abrasion of granular media. Thus the flow of flue gas through perforated vertical wall is obstructed. In addition, the exhaust gas flow also gives rise to pinning of granular media to the perforated vertical wall. Pinning of granules hampers their smooth sliding along the wall. Also, the exhaust gas flow can create some cavities on the vertical perforated wall which disturb flow of granules and decrease their filter efficiency.